Apparatus and Method for Selectively Placing Additives in Wellbore Cement

ABSTRACT

An apparatus for releasing additive into cement pumped into an annular space between a wall of a wellbore and a casing inserted into the wellbore include a cement additive enclosed in a container disposed in the annular space at at least one selected position along the wellbore. The container is configured to release the additive at a selected time.

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority is claimed from U.S. Provisional Application No. 61/267,094 filed on Dec. 7, 2009.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of placing cement in wellbores between a protective pipe or casing and the wall of the wellbore. More specifically, the invention relates to techniques for application of cement additives in wellbore cement at specific locations to provide selected cement properties. The invention relates in particular to a method and apparatus for achieving cementing isolation along the annulus between the wellbore wall and the outside of the casing without the problems of formation breakdown due to high circulating densities and failure to correctly initiate the setting of cement at the desired location in the wellbore.

2. Background Art

Oil and gas wells require effective hydraulic isolation in the annulus (the space between a protective pipe or casing and the wall of the drilled wellbore) between various subsurface formations to provide pressure integrity in the completed wellbore and to prevent contamination of hydrocarbon and fresh water zones by fluid movement between the pipe or casing and the wellbore. Hydraulic isolation is typically obtained by placing cement in the annular space. Achieving proper placement of the cement in the wellbore annuls to provide the required amount of hydraulic isolation requires pumping the cement to the correct place in the annulus and allowing the cement to set and bond to the formation and the pipe. Cement is typically pumped as a high density slurry through the interior of the casing, out the bottom thereof and into the annular space. The high density and viscosity of the typical cement slurry creates high circulating pressures along the wellbore annulus, especially when excess cement slurry must be used to offset the uncertainty in achieving a desired set point of hardened cement. The use of extra volumes of cement slurry creates a higher bottom-hole pressure because of the increased hydrostatic gradient exerted on the formation by the cement slurry. Such high pressure can break down (fracture) certain subsurface formations. Fracturing results in formation damage in sensitive formations, reduction of the available cement in the annulus to provide isolation (cement “tops” ending at greater depth in the annulus than required) and may require complicated and somewhat unreliable cement “squeeze” repairs.

As explained above, one of the major problems in achieving effective cementing isolation in the annulus of a well has been from breakdown (fracturing) of a formation from high circulating fluid pressures caused by the density of cement slurry used for isolation (15 to 16 lb/gal) and the friction produced by the high viscosity of the cement slurry and use of excess volumes of cement to offset uncertainty of the cement set point in the annulus, all, both of which combine to generate effective circulating densities (ECD) and resultant bottom hole pressures that are usually much higher than the density of the drilling fluid (“mud”) used to drill the well. If the pressures produced by the cement slurry ECDs are higher than the fracture pressure of a particular subsurface formation, such formation will break down and cement will be lost into the formation from the annulus. This creates severe damage in almost any formation and losses from the cement volume endanger achieving cement fill and isolation necessary for the particular wellbore design. The problem is worsened in long intervals of cement contact where very large amounts of the cement slurry are needed for coverage using conventional cementing methods.

If cement slurry volumes can be reduced and the pumped slurry could be activated at a precise location in the annulus without additives in the pumped slurry, the job costs would be reduced, the isolation quality would be substantially increased and formation damage from cement would be substantially eliminated. To achieve this goal, addition of additives is necessary where the cement setting reaction is needed.

The transition of pumpable cement slurry to set cement capable of resisting pressure is created by a chemical reaction with the mix water, the completeness of which is related to time and temperature. The reaction time can be modified by additives, but mixing, dilution, loss and inaccuracies of measurement in the mix have created problems when using additives such as cement accelerators and retarders (which modify the time for cement to set). The result is that both accelerators (such as calcium chloride), needed to shorten the set time of the cement, and retarders, (such as lignosulfonate gel) used to lengthen the set time of the cement slurry, are used with a moderate to high degree of risk. Failures are common and can be extremely expensive.

The foregoing additives have been used in encapsulated form and pumped with the cement slurry for some time, but the stability of the encapsulated, coated or otherwise initially segregated additives has always been a problem as the cement slurry goes through a blender, pumps, down thousands of feet of pipe into the well and up the annulus of the pipe and the drilled hole to where it is needed to create isolation. The encapsulation or isolation of the additives is thus subject to failure by the rigors of the pumping system and the potential loss of additives to the formation caused by leakoff or fracturing. The result is an increased risk of failure to place the cement correctly in the wellbore. There is a need for better techniques for selective placement of cement additives in a wellbore.

SUMMARY OF THE INVENTION

An apparatus according to one aspect of the invention for releasing additive into cement pumped into an annular space between a wall of a wellbore and a casing inserted into the wellbore includes a cement additive enclosed in a container disposed in the annular space at at least one selected position along the wellbore. The container is configured to release the additive at a selected time.

A method for releasing an additive to cement pumped into an annular space between a wall of a wellbore and a casing inserted into the wellbore according to another aspect of the invention include disposing the additive in a container in the annular space. Cement is pumped through the wellbore and into the annular space. The additive is released from the container at a selected time.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a vertical wellbore with casing therein and cement additives disposed in the wellbore annulus in casing centralizers.

FIG. 2 shows part of a horizontal wellbore with helical flow inducing centralizers, and additives disposed in the wellbore annulus.

FIG. 3 shows part of another example horizontal wellbore using a sequenced release of additives created by coating these additives with coatings with solubility to different spacer fluids.

DETAILED DESCRIPTION

This invention relates to methods and apparatus that can achieve as-specified set cement solidification producing set-points as designed, with low risk and low cost.

This invention includes placing cement additives, principally, but not limited to accelerators, dispersants, fluid loss additives, or retarders, on the outside of the casing string before the casing is run into the drilled hole. The additive may be encapsulated, coated or held isolated from the circulating fluids used to condition the hole before the cement is pumped into the annulus. The additives may be held on the outside of the casing, the equipment used with the casing including, but not limited to, pockets of additives in protected, sealed or encapsulated areas of centralizers, rough coat encapsulated particles in individual application or groups of particles, adhering to the outer casing wall, devices that dispense solid or liquid additives on demand, etc. Additives located in or near the centralizers may be the most effective, especially if coupled with spiral-flow inducing centralizers that would create mixing of the additive with the cement and generate dispersion of the additive and cement all around the casing.

The usefulness of the method of the invention could be extended by using several additive addition points along the outside of the casing where the encapsulation could be broken or dispersed, thus releasing the additive, by one or more different catalysts. These additive release methods could include reactivity to a preflush ahead of the cement slurry, solution by one of the components of the pumped cement, reaction with high pH of the cement, an exothermic or endothermic reaction or condition of the wellbore with any component of the flush or pumped cement or other physical, electronic or chemical reaction or stimulation.

FIG. 1 shows an example drilled wellbore 2 having a casing 1 inserted therein. Cement may be pumped downward through the casing as indicated by the arrows. The term “pumping cement” as used in the present context is intended to include the cement slurry itself and any fluids pumped before the cement or after the cement. The pumped cement leaves the casing as shown at 3 and travels upwardly through an annular space (“annulus”) 4 between the wellbore 2 and the casing 1. A first casing centralizer of any type known in the art is shown at 5 on the exterior of the casing 1 in a position where additives are not intended to be mixed with the pumped cement. A second casing centralizer, shown at 6, is disposed on the exterior of the casing 1 at a place where it is intended that additives are to be mixed with the pumped cement. The additive may be encapsulated such that the additive is released into annulus 4 at a selected time. Typically the release will take place when the cement slurry is positioned proximate the second centralizer 6.

In another example, shown in FIG. 2, the wellbore 2 may be drilled horizontally. In the example of FIG. 2, a spiral blade centralizer 8 (or suitable vanes) may be placed on the exterior of the casing 1 in a position where additive is intended to be released. Encapsulated additive is shown at 9. In the present example, the pumped cement is induced to flow helically by the blades on the spiral centralizer 8. The helically flowing pumped cement may act to break the encapsulation more effectively than in the example shown in FIG. 1, particularly if the encapsulation is of a type to be broken by solubility of the encapsulated coating with the flush or cement, or erosion effect of the pumped cement on thereon. The helical flow may also be useful to induce mixing the additive with the pumped cement.

In another example, shown in FIG. 3, a first centralizer shown at 6 includes additive disposed in a first type of encapsulation on the centralizer. A second centralizer shown at 7 may include additive disposed in a second type of encapsulation different from the first type. In the example shown in FIG. 3, the type of encapsulation may be selected so that the additives are released at different times or in response to different fluids in the pumped cement.

Specific techniques for placement of cement additives according to the invention include, as non-limiting examples:

(1) Holding the cement additives, principally accelerators, dispersants or fluid loss additives or retarders in a selected position outside the casing (in the annulus) and covering or encapsulating the additives with a coating that can be selectively stripped by preflush ahead of the cement slurry.

(2) Holding the additives in conventional or specially designed equipment on the outside of the casing, that is, a device that contains the additive and is configured to discharge the additive upon a suitable control signal. The control signal may be mechanical (e.g., pressure, impact, etc.) or electrical.

(3) By precisely setting the cement in a specific area with accelerator addition, much smaller cement volumes could be used, which would create lower total ECD and resultant pressures on the formation.

(4) A sequence of additive containers may be disposed along the outside of the casing. The cement slurries and preflushes can be are reactive with a specific type of encapsulating material such that the cement can form set cement isolation “plugs” along the wellbore. In such example, long sections may be allowed to remain open and not cemented, but may be isolated from adjacent zones by the set cement plugs. Such long zones of unset cement could be achieved and ensured in a sequenced cement plug design by releasing “poisons” that prevent cement setting (e.g., sugar), at specific places that would prevent the cement from setting.

(5) Use of the externally released additives and mixing with a spiral-flow inducing centralizer.

(6) Use of the solid or liquid additives added at the desired set-cement point for plugs in both inside and outside the casing string.

(7) The sequence setting of plugs with a single pumping of flush and cement slurries.

(8) Additive release triggering mechanisms by chemical, physical, time and electronic methods.

Apparatus and methods according to the various aspects of the invention may provide improved control over wellbore cement properties, more accuracy in placement of additives and reduced incidence of fracture damage to formations during pumping.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. 

1. An apparatus for releasing additive into cement pumped into an annular space between a wall of a wellbore and a casing inserted into the wellbore, comprising: a cement additive enclosed in a container disposed in the annular space at at least one selected position along the wellbore, wherein the container is configured to release the additive at a selected time.
 2. The apparatus of claim 1 wherein the container comprises a material that is chemically reactive with a component of pumped cement.
 3. The apparatus of claim 2 wherein the chemical reaction comprises one of an exothermic reaction, reaction to pH of a component of the pumped cement and solution.
 4. The apparatus of claim 1 wherein the container is configured to discharge the cement additive therefrom on application of a control signal.
 5. The apparatus of claim 4 wherein the control signal is at least one of mechanical and electrical.
 6. The apparatus of claim 1 further comprising a casing centralizer disposed proximate the container.
 7. The apparatus of claim 6 wherein the centralizer comprises spiral blades or vanes.
 8. A method for releasing an additive to cement pumped into an annular space between a wall of a wellbore and a casing inserted into the wellbore, comprising: disposing the additive in a container in the annular space; pumping cement through the wellbore and into the annular space; and releasing the additive from the container at a selected time.
 9. The method of claim 8 wherein the container comprises a material that is chemically reactive with a component of pumped cement.
 10. The method of claim 9 wherein the chemical reaction comprises one of an exothermic reaction, reaction to pH of a component of the pumped cement and solution.
 11. The method of claim 8 wherein the container is configured to discharge the cement additive therefrom on application of a control signal.
 12. The apparatus of claim 11 wherein the control signal is at least one of mechanical and electrical.
 13. The method of claim 8 further comprising disposing a casing centralizer disposed proximate the container.
 14. The method of claim 13 wherein the centralizer comprises spiral blades or vanes 